Nanoscale structure damage in irradiated W-Ta alloys for nuclear fusion reactors

In this project, we have assessed the structural tolerance of advanced refractory alloys to simulated nuclear fusion reactor environments, by using intense proton beams to mimic fusion neutron damage and analysing the proton damaged structures using in-situ/ex-situ transmission electron microscopy and nano-hardness measurements. Refractory metals such as tungsten or tantalum, and their binary alloy combinations, are considered as promising structural materials to withstand the unprecedented high heat loads and fast neutron/helium fluxes expected in future magnetically-confined fusion reactors. Tungsten is currently the frontrunner for the production of plasma-facing components for fusion reactors. The attractiveness of tungsten as structural material lies in its high resistance to plasma-induced sputtering, erosion and radiation-induced void swelling, together with its thermal conductivity and high-temperature strength. Unfortunately, the brittle nature of tungsten hampers the manufacture of reactor components and can also lead to catastrophic failure during reactor operations. We have focused on two potential routes to enhance the ductility of tungsten-containing materials, namely alloying tungsten with controlled amounts of tantalum, and using alternatively tantalum-based alloys containing specific tungsten additions, either as a full-thickness structural facing material or as a coating of first wall reactor components. The aim was to investigate the formation and evolution of radiation-induced damaged structures in these material solutions and the impact of those structures on the hardness of the material. The main results of this work are: (1) the addition of 5wt%Ta to W leads to saturation in the number density and average dimensions of the radiation-induced a/2 dislocation loops formed at 350C, whereas in W the loop length increases progressively and evolves into dislocation strings, and later into hydrogen bubbles and surface blisters, (2) the recovery behaviour of proton irradiated W5wt.%Ta alloy is characterized by dislocation loop growth at 600-900C, whereas voids form at 1000C by either vacancy absorption or loop collapse, (3) the presence of radiation-induced a loops at 590C in Ta hinders the formation and ordering of voids observed with increasing damage levels at 345C, (4) the addition of 5-10wt.%W to Ta delays the evolution of a/2 dislocation loops with increasing damage levels, and therefore the appearance of random voids. These results expand the composition palette available for the safe selection of refractory alloys for plasma facing components with enhanced, or at least predictable, tolerance to the heat-radiation flux combinations expected in future nuclear fusion plants.